Multicellularity represents one of the major transitions in the evolution of biologic complexity.1 Bacterial species such as Myxococcus xanthus exhibit characteristics of multicellular organisms, including cooperative behavior among individuals and coordinated cell-cell attachments.2 Here it is shown that pleomorphic cells derived from mammalian tumor exhibit morphologic and genetic characteristics of both prokaryotic and eukaryotic cells, among the most conspicuous of which is the resemblance to prokaryotic cells at the unicellular level, combined with the ability to self-organize in vitro into multicellular, mammalian eukaryotic tissue-like patterns consisting of tissue-like sheets, capillary-like networks, and trabecular (spongy) bone-like structures. This multicellularity is unlike any previously reported.